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1.
Torsional forced-oscillation and microcreep methods have been employed in a study of the viscoelastic behaviour of fine-grained
polycrystalline olivine at high temperatures (to 1300 °C), seismic frequencies and low strain amplitudes. The Fo90 specimens are of low porosity and low dislocation density. They vary in mean grain size from 8 to 150 μm and contain only
trace amounts (≪0.1 vol%) of quenched melt glass. For T ≤ 900 °C, their behaviour is essentially elastic and the shear modulus G closely approaches that expected for a dense polycrystal from single-crystal elasticity data – confirming the suppression
of thermal microcracking in␣this study. At higher temperatures, pronounced absorption-band dissipation and associated dispersion␣of
the shear modulus provide evidence of linear viscoelastic behaviour. Both recoverable (anelastic) and permanent (viscous)
strains are involved and the proportion of the latter increases with increasing temperature and decreasing frequency. Comparison
of the results for the three specimens provides a clear indication that the viscoelastic behaviour, attributed to diffusional
processes, is grain-size-sensitive with the dissipation and associated dispersion increasing with decreasing grain size. Both
elastically accommodated and diffusionally accommodated grain-boundary sliding appear to be implicated.
Received: 29 September 2000 / Accepted: 7 May 2001 相似文献
2.
F. Farges 《Physics and Chemistry of Minerals》2001,28(9):619-629
Fe–K edge XAFS spectra (pre-edge, XANES and EXAFS) were collected for eight grandidierites from Madagascar and Zimbabwe,
as well as for Fe(II) and Fe(III) model compounds (staurolite, siderite, enstatite, berlinite, yoderite, acmite, and andradite).
The pre-edge spectra for these samples are consistent with dominantly 5-coordinated ferrous iron. The analysis of the XANES
and EXAFS spectra confirms that Fe(II) substitutes for Mg(II) in grandidierite, with a slight expansion of the local structure
around Mg by ∼2%. In addition, ferric iron was also detected in some samples [5–10 mol% of the total Fe or 500–1100 ppm Fe(III)].
Based on theoretical calculations of the EXAFS region, Fe(III) appears to be located in the 5-coordinated sites of Mg(II)
or in the most distorted 6-coordinated sites of Al (depending on the sample studied). Special attention is therefore required
when using grandidierite as a model for ferrous iron in C3v geometry, because of the possible presence of an extra contribution related to Fe(III). This additional contribution enhances
significantly the Fe–K pre-edge integrated area [+40% for 1000 ppm Fe(III)]. Therefore, only a few grandidierite samples can
be used as a robust structural model for the study of the Fe(II) coordination in glasses and melts.
Received: 26 June 2000 / Accepted: 19 February 2001 相似文献
3.
N. Shimobayashi A. Miyake M. Kitamura E. Miura 《Physics and Chemistry of Minerals》2001,28(9):591-599
Phase transition between low-temperature clinoenstatite (LT-CEn) and high-temperature clinoenstatite (HT-CEn) was studied
by using molecular dynamics (MD) simulations, based on empirical potential parameters. Starting from LT-CEn, the MD calculations
were carried out at atmospheric pressure and at elevated pressures (1–6 GPa). At elevated temperatures the transformation
from the starting LT-CEn to HT-CEn occurred at any pressure. It was confirmed that the HT-CEn has the same space group C2/c as diopside but the M2 site is six-coordinated, unlike diopside. A significant difference in the MD-simulated cell volumes
between LT-CEn and HT-CEn was also observed, showing a first-order transition. In addition, there were some temperature ranges
where LT-CEN and HT-CEn would be coexistent and very small thermal hystereses between increasing and decreasing temperatures
during the transition. These behaviors are consistent with the characteristic of a thermoelastic-martensitic transformation.
The phase boundary between LT-CEn and HT-CEn was determined for the first time. Its positive dT/dP slope strongly shows that the high-pressure clinoenstatite is a significantly distinct phase from HT-CEn although the both
phases have the same space group, C2/c.
Received: 8 November 2000 / Accepted: 28 April 2001 相似文献
4.
We present Raman and infrared spectra of gypsum to 21 GPa at 300 K. Our measurements encompass the internal modes of the
(SO4)−4 group that lie between 400 and 1150 cm−1, hydroxyl-stretching vibrations between 3200 and 3600 cm−1, and a libration and bending vibrations of the molecular H2O group. All vibrations of the sulfate group have positive pressure shifts, while the hydroxyl-stretching and -bending vibrations
have a mixture of positive and negative pressure shifts: the effect of pressure on the hydrogen bonding of the water molecule
thus appears to be complex. Near 5 GPa, the two infrared-active bending vibrations of the water molecule coalesce, and the
morphology of the hydroxyl-stretching region of the spectrum shifts dramatically. This behavior is consistent with a pressure-induced
phase transition in gypsum in the vicinity of 5–6 GPa, which is observed to be reversible on decompression to zero pressure.
The spectral observations are consistent with the onset of increased disorder in the position of the water molecule in gypsum:
the sulfate vibrations are largely unaffected by this transition. The Raman-active symmetric stretch of the sulfate group
undergoes an apparent splitting near 4 GPa, which is interpreted to be produced by Fermi resonance with an overtone of the
symmetric bending vibration. The average mode Grüneisen parameter of the 20 vibrational modes we sample is less than 0.05,
in contrast to the bulk thermal Grüneisen parameter of 1.20. Accordingly, the vibrations of both water and sulfate units within
gypsum are highly insensitive to volumetric compaction. Therefore, in spite of the changes in the bonding of the water unit
near 5 GPa, metastably compressed gypsum maintains strongly bound molecular-like units to over 20 GPa at 300 K.
Received: 31 July 2000 / Accepted: 5 April 2001 相似文献
5.
本文研究了河北省涉县符山铁矿夕卡岩中石榴石的双折射性。该石榴石属钙铝-钙铁榴石系列,有较强的双折射性,重折率最大值约为0.009。加热可使双折射性减弱甚至消失。依形成温度,可将石榴石分为两群,重折射率相差一、两个数量级,形成温度较低者其双折射性强。在每一群中,重折率与分子式中八面体位置 Al 含量成上弯曲线关系,最强双折射大致在 And65 Gro35成分处。X 光单晶衍射照相出现异常,衍射点变成由数个点组成的拉长的线,这些点沿一定方向排列,强度和数目有变化,可能是因为结构中八面体位置上 Fe3+/Al 有序引起的。有序引起石榴石对称性降低,变为二轴晶,可以较好地解释石榴石双折射性的成因及其变化规律。本文还讨论了环带与双折射性的关系。 相似文献
6.
S. Boursiquot M. Mullet M. Abdelmoula J.-M. Génin J.-J. Ehrhardt 《Physics and Chemistry of Minerals》2001,28(9):600-611
The gradual oxidation of dry mackinawite (tetragonal FeS1? x ) has been studied using X-ray diffraction (XRD), transmission electron microscopy (TEM), transmission Mössbauer spectroscopy (TMS) and X-ray photoelectron spectroscopy (XPS). The initial material and samples exposed to the air (5?min to 6?months) have been analysed. Diffraction patterns showed the slow disappearance of mackinawite with time with concomitant appearance of greigite (Fe3S4) and elemental sulphur (S(0)) as well as iron (oxyhydr)oxides, i.e. magnetite (Fe3O4) and probably goethite (α-FeOOH). After 6 months' air exposure, mackinawite and also greigite were entirely converted into elemental sulphur and iron (oxyhydr)oxide(s), indicating that greigite was an intermediate reaction product. Mössbauer spectra of samples oxidized in air appeared rather complex for interpreting what was easily conceivable in view of the association of several phases, as revealed by the diffraction patterns. The low-temperature Mössbauer spectrum obtained after 6?months air exposure was attributed to magnetite, although a mixture of magnetite and goethite was not completely excluded. XPS iron and oxygen data confirmed the formation of Fe(III) (oxyhydr)oxides at the surface after an induction period. Sulphur spectra demonstrated various oxidation states from S(-II) (monosulphide) to S(VI) (sulphate) for the longest experiments. Mackinawite in these experiments reacted mainly with adsorbed O2 to form elemental sulphur and magnetite. Additionally, sufficient sulphur was generated to react stoichiometrically with mackinawite to produce greigite. Finally, greigite, in the longest experiments, was transformed into elemental sulphur and magnetite. 相似文献
7.
H. W. Nesbitt A. G. Schaufuss G. M. Bancroft R. Szargan 《Physics and Chemistry of Minerals》2002,29(1):72-77
Synchrotron excited X-ray photoelectron spectra (SXPS) of hexagonal pyrrhotite reveal three distinct Fe 3d-derived photopeaks within its outer valence band. The t
2gα band (majority spin) is centered at about 2.5 eV, the e
g
α band at about 1.0 eV and the t
2gβ (minority spin) contribution at about 0.25 eV. From these data the ligand field splitting energy is 1.5 (±0.2) eV and the
majority spin pairing energy is 2.25 (±0.2) eV. These are the first such XPS measurements for this mineral. S 3p-derived bonding and non-bonding bands are identified, with the former centred at about 6.5 eV and the latter near 4.5 eV.
The XPS results are remarkably consistent with SCF-Xα scattered wave molecular orbital calculations. Although the calculations
and the collected spectra are consistent, they differ from a recent interpretation of the pyrrhotite valence band. An explanation
for the discrepant results is provided. Auger resonant enhancement of Fe 3d photopeaks at 60 eV photon energy results in the
t
2gα emission (at 2.5 eV) being strongly enhanced and broader than the t
2gβ emission (0.25 eV). The explanation of these observations requires the presence of weak Fe–Fe π and π* crystal (molecular) orbitals located near 2.5 eV, and separated by no more than about 0.5 eV. The π-bonded crystal orbitals
are derived from weak mixing of adjacent Fe t
2g atomic orbitals along the c crystallographic axis.
Received: 15 June 2000 / Accepted: 11 June 2001 相似文献
8.
The structural behavior of synthetic gahnite (ZnAl2O4) has been investigated by X-ray powder diffraction at high pressure (0–43 GPa) and room temperature, on the ID9 beamline
at ESRF. The equation of state of gahnite has been derived using the models of Birch–Murnaghan, Vinet and Poirier–Tarantola,
and the results have been mutually compared (the elastic bulk modulus and its derivatives versus P determined by the third-order Birch–Murnaghan equation of state are K
0=201.7(±0.9) GPa, K
′
0=7.62(±0.09) and K
″
0=−0.1022 GPa−1 (implied value). The compressibilities of the tetrahedral and octahedral bond lengths [0.00188(8) and 0.00142(5) GPa−1 at P=0, respectively], and the␣polyhedral volume compressibilities of the four-␣and␣sixfold coordination sites [0.0057(2) and
0.0041(2) GPa−1 at P=0, respectively] are discussed.
Received: 15 January 2001 / Accepted: 23 April 2001 相似文献
9.
An olivine grain from a peridotite nodule 9206 (Udachnaya kimberlite, Siberia) was investigated by TEM methods including
AEM, HRTEM, SAED and EELS techniques. A previous study of the 9206 olivine sample revealed OH absorption bands in the IR spectrum
and abundant nanometer-sized OH-bearing inclusions, of hexagonal-like or lamellar shape. Inclusions, which are several hundred
nm in size, consist of 10 ? phase, talc and serpentine (chrysotile and lizardite). The lamellar (LI) and hexagon-like small
inclusions of several ten nm in size (SI) are the topic of the present paper. AEM investigations of the inclusions reveal
Mg, Fe and Si as cations only. The Mg/Si and Fe/Si atomic ratios are lower in the inclusions than in the host olivine. The
Si concentration in the olivine host and both lamellar inclusions and small inclusions is the same. A pre-peak at 528eV was
observed in EEL spectra of LI and SI, which is attributed to OH− or Fe3+. From these data it is concluded that there is a OH−- or Fe3+-bearing cation-deficient olivine-like phase present.
HRTEM lattice fringe images of LI and SI exhibit modulated band-like contrasts, which are superimposed onto the olivine lattice.
Diffraction patterns (Fourier-transforms) of the HREM images as well as SAED patterns show that the band-like contrasts in
HRTEM images of the inclusions are caused by periodic modulations of the olivine lattice. Three kinds of superperiodicity
in the olivine structure such as 2a, 3a and 3c, were observed in SAED patterns. The corresponding olivine supercells labelled
here as Hy-2a, Hy-3a and Hy-3c were derived. The M1-vacancies located in the (100) and (001) octahedral layers of the olivine
lattice are suggested to form ordered arrays of planar defects (PD), which cause the band-like contrasts in HRTEM patterns
as well as the superperiodicity in the SAED patterns.
The vacancy concentrations as well as the chemical composition of Hy-2a, Hy-3a and Hy-3c olivine supercells were calculated
using crystal chemical approaches, assuming either {(OH)<
O−V"
Me−(OH)<
O}↔, or {F
e
<
Fe
−H
Me
′}↔ or {2F
e
<
Fe
−V
Me
"}↔ point defect associates. The calculated theoretical compositions Mg1.615Fe+2
0.135v0.25SiO4H0.5 (Hy-2a) and Mg1.54Fe2+
0.12v0.33SiO4H0.66 (Hy-3a and Hy-3c) are in a good agreement with the AEM data on inclusions. Hy-2a, Hy-3a and Hy-3c are considered to be a
hydrous olivine with the extended chemical formula (Mg1-yFe2+
y)2−xvxSiO4H2x. The crystal structure of hydrous olivine is proposed to be a modular olivine structure with Mg-vacant modules. The crystal
chemical formula of hydrous olivines in terms of a modular structure can be written as [MgSiO4H2] · 3[Mg1.82Fe0.18SiO4] for Hy-2a, [MgSiO4H2] · 2[Mg1.82Fe0.18SiO4] for Hy-3a and Hy-3c.
Hydrous olivine is suggested to be exsolved from the olivine 9206, which has been initially saturated by OH-bearing point
defects. The olivine 9206 hydration as well as the following exsolution of hydrous olivine inclusions is suggested to occur
at high pressure-high temperature conditions of the upper mantle.
Received: 15 January 2001 / Accepted: 2 July 2001 相似文献
10.
11.
An automated method for the calculation of P–T paths based on garnet zoning is presented and used to interpret zoning in metapelitic schist from the southern Canadian Cordillera. The approach adopted to reconstruct the P–T path is to match garnet compositions along a radial transect with predictions from thermodynamic forward models, while iteratively modifying the composition to account for fractional crystallization. The method is applied to a representative sample of garnet‐ and staurolite‐bearing schist from an amphibolite facies Barrovian belt in the southern Canadian Omineca belt. Garnet zoning in these schists is concentric and largely continuous from core to rim. Three zones are present, the first two of which coincide with sector‐zoned cores of garnet crystals. Similar zoning is developed in rocks that contain or lack staurolite, respectively, suggesting garnet growth was restricted to the initial part of the prograde P–T path prior to the development of staurolite. Growth zoning in large garnet crystals has not been significantly modified by diffusion. This interpretation is based on zoning characteristics of garnet crystals and is further supported by results of a forward model incorporating the effects of simultaneous fractional crystallization and intracrystalline diffusion. The P–T path calculated for this rock includes an initial, linear stage with a high dP/dT, and a later stage dominated by heating. The approach adopted in this study may have application to other garnet‐bearing rocks in which growth zoning is preserved. 相似文献
12.
Tsuyoshi Morohashi Shohei Banno Masao Yamasaki 《Contributions to Mineralogy and Petrology》1974,47(3):187-196
Single whole crystals of calcic augite from M. Etna, Sicily, which display both sector and oscillatory zoning, have been serially sectioned and the form of the sectors mapped out. The principal intersectorial chemical variation is shown to be that of Ti and Al by as much as 39% (cation %) change in each, and the least variation, that of Ca (showing a 0.77 to 3.20% change). For each of three isochronous surfaces the ratio of Ti∶Al is shown to be individually constant no matter what the sector and this is thought to indicate near equilibrium conditions at the crystal/liquid interface. In addition to sector zoning these crystals display two types of concentric banding, having dimensions of about 0.25 mm, 20 μm. The 20 μm banding is shown to be both optically and chemically oscillated. Relative diffusion rates of cationic species in the melt, and growth rates of crystal faces are thought to play an important role in the formation of the oscillations, and the sector zoning. 相似文献
13.
白云石成因研究新方法--白云石晶体结构分析 总被引：6，自引：0，他引：6
传统的白云石研究方法对于白云石成因分析具有多解性,对于已有白云石成因模式的套用或重新建立新的白云石成因模式将白云石的形成机理过于简单化、模式化。白云石的晶体结构保存了晶体的形成环境、结晶速度、晶体生长与变化特征、流体特征的证据,白云石晶体结构分析是进行白云石成因分析的有效手段,之前很少有研究者从晶体结构角度进行白云石成因分析。在岩石学和地球化学研究的基础上,本文对四川盆地灯影组-寒武系鞍状白云石、纤状白云石、残余颗粒细晶白云石、孔洞充填粗晶白云石和泥晶白云石五种白云石进行了微组构取样,并利用X衍射仪、透射电镜等晶体结构研究手段,从晶体结构角度对五种白云石组构的有序度、晶胞参数、晶格条纹、晶面间距、晶格缺陷等晶体结构参数进行了差异性研究,分析了它们不同的形成环境和成岩演化特征,初步建立了不同类型白云石晶体结构判识标志。 相似文献
14.
The Lower Ordovician, Upper Knox Group rocks (the Kingsport and Mascot formations) in the Copper Ridge district consist predominantly of fine-grained dolostones, medium and coarser grained dolostones, and limestones. Dolomite crystals of medium and coarser grained dolostones show up to eight cathodoluminescent zones of variable width and intensity. Electron microprobe analyses indicate that the zoning is related to variation in Fe/Mn ratios, the brighter luminescent zones corresponding to lower ratios. Superposed on this growth zoning is a compositional zoning characterized by a general increase in Fe from core to rim of individual dolomite crystals.Field and petrographic studies (Churnet, 1979; Churnet et al., 1981) indicate that the fine-grained dolostones formed in supratidal to upper intratidal environments, whereas the precursor lime muds of the limestones as well as of the medium and coarser grained dolostones formed in shallow subtidal to lower intertidal environments. The large areal extent of the dolostones must have required a regionally abundant source of Mg such as marine water. Yet, both limestones and dolostones have low Na and Sr contents suggestive of their formation in solutions more dilute than normal marine water. It is proposed that the fine-grained dolostones formed by aggradation of initially very fine-grained dolostones in presence of fresh water, and that the limestones stabilized and the medium and coarser grained dolostones formed in environments of mixed marine and fresh waters. Considered in the light of ordering of partition coefficients, such a mixing model can account for the observed correlation pattern of trace elements (especially, SMn and SrFe) as well as the Fe distribution in the zoned dolomite crystals. Variation of the partition coefficient of Mn due to fluctuations in the relative proportions of fresh and marine waters in the diagenetic solution may explain the different Fe/Mn ratios observed in the growth zones (luminescence bands) of zoned dolomite crystals. 相似文献
15.
The relationship between the distribution of nitrogen impurity centres in diamond crystals and their internal structure and mechanism of growth 总被引：1，自引：0，他引：1
Fifty diamond crystals of different morphological types (octahedra, dodecahedroids, cubes and single tetrahexahedroid) with differing internal structures were examined using methods of cathodoluminescence (CL), anomalous birefringence and local infrared (IR) analysis. The main objective of the study was to examine the regularities of nitrogen impurity distribution in diamond with differing internal structures. Almost all the analyzed octahedra, as well as dodecahedroids with zonal structures and the blocky dodecahedroids, are characterized either by nearly isothermic growth conditions or by a decrease in formation temperature during the crystallization process. In contrast to zoned octahedra and dodecahedroids, dodecahedroids with zonal–sectorial and sectorial internal structures show a notably different distribution of nitrogen defects, with Ntot generally decreasing from crystal cores to marginal areas, and degree of nitrogen aggregation increasing in the same direction. From this, it would follow that in these crystals, the temperature of diamond formation of the outer crystal zones is approximately 40–50 °C higher than that of the inner zones. The same result (15 to 80 °C) was obtained for diamond crystals with cubic habit, which generally show a fibrous internal structure, reflecting normal mechanisms of growth. The anomalous distribution of nitrogen centres in diamond crystals that grew through the normal mechanism, with a high rate of growth and in an oversaturated medium, might point to non-equilibrium relationships between the concentrations of different nitrogen centres. It is likely that in crystals of this type, the rate of growth is higher than the rate of structural nitrogen aggregation. Thus, it appears that in these peculiar crystals of diamond we deal with non-equilibrium concentrations of nitrogen B centres and, consequently, with anomalous, non-actual diamond formation temperatures. 相似文献
16.
An analytical and experimental study of zoning in plagioclase 总被引：1，自引：0，他引：1
A detailed electron microprobe study has been conducted on natural and experimentally grown zoned plagioclase feldspars. Discontinuous, sector, and oscillatory chemical zoning are observed superimposed on continuous normal or reverse zoning trends. The relative accuracy of 3 element (Na, Ca, K) microprobe traverses was found statistically to be 2 mole percent. Comparison of microprobe data on natural zoned plagioclase with zoned plagioclase grown in controlled experiments has shown that it may be possible to distinguish zonal development resulting from physio-chemical changes to the bulk magma from zoning related to local kinetic control on the growth of individual crystals. 相似文献
17.
Mizuhiko Akizuki 《Lithos》1981,14(1):17-21
At a low degree of supersaturation a crystal forming from solution grows by repeated addition of layers to the crystal faces. A three-dimensional structure is therefore formed by the stacking of two-dimensional nets. The two-dimensional atomic arrangement exposed on the growing faces and its symmetry controls the ordering of cations and/or anions added to the surface. The distribution of Al and Si seems to be particularly prone to ordering of this kind. In chabazite the optical properties corresponding to the degree of ordering differ from growth sector to growth sector produced on the vicinal faces {hkill} of growth hillocks. The optically triclinic sectors in chabazite form on a vicinal face which inclines to the morphological mirror plane and the c-axis, and the mirror plane of the growth hillock becomes the twin plane of the sector. Thus, chabazite showing rhombohedral form consists of six twinned sectors corresponding to six {10ulbar;11} faces in which some small twinned sectors correlated with the symmetrical vicinal faces can be seen under the optical microscope. 相似文献
18.
The time and temperature evolution of twinning in cordierite is simulated using three computer models. The orientation of
walls between twin domains in natural cordierite follows mainly the ferroelastic pattern which minimises the strain energy
of the walls between twin-related domains. Such ferroelastic twinning is simulated in an elastic three-states Potts model
in which each structural six-membered ring is represented by a three state pseudo-spin. The resulting twin pattern in a sample
with 3169 structural rings shows sector trilling and fine scale ferroelastic wall patterns which coarsen with increasing annealing
time. The poorly defined wall directions observed in cordierite were found to be related to twin walls which do not minimise
the strain energy. Instead, these walls are located along the corners of pseudo-hexagonal rings and appear as the consequence
of local rather than global interatomic interactions. Simulations using two-dimensional (38028 atoms) and three-dimensional
(408 228 atoms) structural models show a predominance of these topological walls over the strain walls at early stages in
the ordering process. The domain structure in the simulation is patchy rather than corresponding to repeated stripe structures
found in other ferroelastic and co-elastic materials. In all models, a strong tendency for sector trilling is observed. In
kinetic tweed patterns a novel 60o tweed is found at atomic length scales while the usual strain-mediated 90o tweed appears
at mesoscopic length scales. An unusual surface tension effect in domain formation and ’writhing’, fluid-like motion was found
in the three-dimensional structural model. This motion, along with the existence of non strain-mediated walls may contribute
to cordierite’s poorly defined domain wall directions at the early stages of domain coarsening.
Received: 11 March 1998 / Revised, accepted: 28 August 1998 相似文献
19.
Optical absorption spectra (OAS) of synthetic single crystals of the solid solution spinel sensu stricto (s.s.)–magnesioferrite, Mg(Fe3+Al1???y)2O4 (0?y?≤ 0.3), have been measured between 12 500 and 28 500?cm?1. Chemical composition and Fe3+ site distribution have been measured by electron microprobe and Mössbauer spectroscopy, respectively. Ferric iron is ordered to the tetrahedral site for samples with small magnesioferrite component, and this ordering is shown to increase with magnesioferrite component. The optical absorption spectra show a strong increase in band intensities with Fe3+→Al substitution. Prominent and relatively sharp absorption bands are observed at 25 300 and 21 300?cm?1, while less intense bands occur at 22 350, 18 900, 17 900 and 15 100?cm?1. On the basis of band energies, band intensities and the compositional effect on band intensity, as well as structural considerations, we assign the observed bands to electronic transitions in IVFe3+–VIFe3+clusters. A linear relationship (R 2= 0.99) between the αnet value of the absorption band at 21 300?cm?1 and [IVFe3+]?·?[VIFe3+] concentration product has been defined: αnet=2.2?+?15.8 [IVFe3+]?·?[VIFe3+]. Some of the samples have been heat-treated between 700 and 1000?°C to investigate the relation between Fe3+ ordering and absorption spectra. Increase of cation disorder with temperature is observed, which corresponds to a 4% reduction in the number of active clusters. Due to the high spatial resolution (??～?10?μm), the OAS technique may be used as a microprobe for determination of Fe3+ concentration or site partitioning. Potential applications of the technique include analysis of small crystals and of samples showing zonation with respect to total Fe3+ and/or ordering. 相似文献
20.
Synthetic (SiGe)-pyroxenoids are often observed to have superperiods. Whether or not these superperiods grow in connection
with Si-Ge ordering is a fundamental question. The size difference between Ge and Si tetrahedra leads to CdGeO3 having the pyroxmangite structure, whereas CdSiO3 has the wollastonite structure. Consequently, considerable strain is expected for a pyroxenoid with a disordered Ge-Si distribution.
A 29Si MAS-NMR study of a Cd-pyroxmangite of nominal composition Cd7[Ge6Si]O7 points to considerable Si clustering, probably as GeSiSiSiGe triples corresponding to the wollastonite-like units of the
siebener chain. It is proposed that such ordering relieves strain within the tetrahedral chain. Residual strain is also relieved
by insertion of an extra pair of GeO4 tetrahedra into the pyroxene-like component of the siebener-chain units, leading to single neuner-chain units which terminate
the superperiods. This growth pattern is cyclic. The presence of several types of superperiod may reflect similar energetics
for different Ge-Si-ordering patterns within the siebener chains. Ordering of Si-rich unit cells and of unit cells having
no Si is proposed as the reason for the occurrence of the superperiods in Cd-pyroxmangite.
Received: 25 August 1999 / Accepted: 19 May 2000 相似文献